DRV8889-Q1EVM: it is an inquiry regarding the output current when driving a stepping motor.

Hello Hoyoung,

Thank you for your post and thanks for sharing the current waveforms. Based on the waveforms it appears you are using full-step mode. Like you mentioned, at high speeds the current will not rise fast enough to reach ITRIP level set by VREF and result in distortion not only due to inductance of the coils but also the back EMF. Back EMF is the primary contributor. The back EMF is a sinusoidal voltage opposing the voltage applied across the coils by the driver. Depending on the load on the stepper motor there is a phase difference between the back EMF and the coil current, see page-36 of the datasheet. This phase difference causes ups and downs in the coil current causing the waveform captured with 5400 RPM. (is this accurate calculation for RPM? such high speeds are unusual in stepper applications. Steppers are not designed for high RPM\. Just a note.). If you load the motor you can notice the waveform would change and when you stall the motor the waveform will become normal because back EMF turns 0.

The DRV8889-Q1 does not have a boost voltage for VM. The charge pump boost is used for generating the voltage necessary for driving the HS output N-FET. It must not (cannot) be disabled while the driver is in operation. The charge pump is disabled when the device is in sleep mode. 

To have cleaner current waveform at higher speeds the VM supply must be increased from 12 V to a higher value enough to overcome the back EMF voltage. Having a distorted waveform is a non issue as long as the motor does not skip steps. It is expected behavior. If you're using the stall detection feature of the device it may be impacted with distorted current waveform and likely will not work as intended.

Regards, Murugavel   

Hello Murugavel

I responded to your email (with additional questions) on November 16th.
The email address is "noreply-e2e@mail.ti.com".

First inquiry
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Depending on the load on the stepper motor there is a phase difference between the back EMF and the coil current, see page-36 of the datasheet. <=== your answer
================
1-1. I don’t understand what’s on page 36. I would like to request a more detailed explanation.

1-2. Unloaded stepper Motor, partially loaded stepper motor, what does the Fully loaded stepper motor image mean?

- If the back-EMF is too large, does this mean that the drive controls to input the current to the limit (up to the current limit condition) through phase control of the current input the coil?

- Does the drive control phase of current at any speeds? If the coil current does not reach the current limit due to back-EMF, is phase control performed?

Second inquiry

We are developing a high-speed stepping motor and matching FEM analysis with actual measurement conditions.

When performing FEM analysis at high speed (5400 [rpm]), the coil current can be confirmed to be much lower than the limiting current as the speed increases. However, the experimental results show a very different waveform of the coil current unlike the FEM analysis. The current waveform reaches the limit current from the beginning and almost reaches the limit current at the end even. What causes the current waveforms in FEM analysis and experiment to be different?

Third inquiry
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To have cleaner current waveform at higher speeds the VM supply must be increased from 12 V to a higher value enough to overcome the back EMF voltage. <=== your answer
============
Isn't the current waveform at 5400rpm a cleaner waveform?
What problems can occur with the 5400rpm current waveform?

Does cleaner current waveform refer to the 3500rpm waveform?

Please let me know, some advice.

Regards, Hoyoung Mun

Hello Hoyoung,

Thanks for reposting your email questions in this forum. Please note "noreply-e2e@mail.ti.com" email is a one way email meaning it will not receive any replies hence "noreply". Sorry, we did not receive your email. This forum is the conversation platform for our discussion regarding your inquiry. Please see below my responses for your three inquiries.

First inquiry:

1-1 Describes behavior of a stepper motor. The red wave form is the actual current waveform of a stepper motor. Here microstepping is shown. For full step and half step this will be a square wave typically. The blue wave is the back EMF which will be a continuous sinusoidal regardless of the step mode. 

1-2 By load I meant the mechanical load on the motor. For a mechanically unladed free running motor (nothing connected to its output shaft) the phase difference between back EMF and the drive current waveform will be close to 90 degrees. When the motor is fully loaded (about to stall under the mechanicall load) the phase angle will be close to 0 degrees. The back EMF voltage opposes the applied voltage and may result in a waveform like you had (shown below). This pattern will change when the motor is mechanically loaded because the phase angle of the back EMF changes. This is what I meant. If you increase the VM supply voltage high enough this distortion will disappear. This is how a stepper drive is expected to behave. The higher the speed of the motor the higher the back EMF amplitude. This causes the current distortion is VM is not sufficient to overcome the back EMF and provide enough voltage across the coil to pump in the target full scale current IFS.

Second inquiry:

I assume you were referring to a software analysis using mathematical model. How did your model get proper back EMF modeling? Did you get the back EMF constant of the motor from the vendor. This parameter is not commonly available. Typically you may have to model this yourself to perform accurate simulation. You asked "What causes the current waveforms in FEM analysis and experiment to be different?". My guess is you used only the L/R time constant in modelling and did not account for back EMF sinewave generated opposing the applied motor voltage. Like I mentioned in the first post the back EMF opposes the applied voltage across the coil. It is 0 V when the motor is standstill and increases proportional to the motor speed. 

Third inquiry

I referred to an undistorted waveform as a cleaner waveform. You are correct, in your application it would mean like the waveform at 3500 RPM.

Regards, Murugavel